This invention relates to local area networks; and in particular it relates to methods of recovering from various errors that occur in the messages that are transmitted through such networks.
Basically, a local area network is comprised of a plurality of stations, each of which have two input-output (I/O) ports that are intercoupled in a loop. That is, one I/O port of station #1 is coupled to one I/O port of station #2; the other I/O port of station #2 is coupled to one I/O port of station #3; . . . etc.; and the second I/O port of the last station is coupled to the second I/O port of the first station. Messages which are spaced by idle characters are normally sent around the loop through all the stations in one direction. If, however, the coupling between two stations breaks, then the messages are "looped-back" at the breakpoint through all the stations in the opposite direction.
In the prior art, a network of this type is described, for example, in the U.S. Pat. No. 4,519,070 issued May 21, 1985 to John L. Bell and assigned to the present assignee. This patent describes and claims a method of initializing the transmission of messages through the network starting from a state in which all of the stations are powered on one-at-a-time in a random sequence. Also, the patent describes a method of rerouting the messages through the stations when the coupling between two stations breaks.
However, many types of error conditions can occur in a local area network besides a broken interstation coupling, and these errors are not considered in the Bell patent. For example, one of the stations can become "stuck" in a particular state in which it continuously sends messages or idle characters and never stops. Also, a station may erroneously send messages in nonconformance with some pre-established format. Such transmission errors typically are very infrequent, but when they do occur they render the entire network inoperable if no corrective action is taken.
Accordingly, a primary object of the invention is to provide a method of operating a station in a local area network in which transmission errors of the above type, and others, are detected and recovered from.